Photographic printing exposure control



Sept. 25, 1956 R. J. HORAK 2,764,060

PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Filed Sept. 25, 1952 8 Sheets-Sheet 1 UM-MINIMUM DENSITY MEASUREMEN C0 CATHODE L FOLLOWER PHFTO E-Ecm DETECTOR RELAYS 9 L1 m H, PRINT ll? L POWER MACHINE SUPPLY ELECTRICAL EQUIPMENT IN V ENTOR ROBERT J. HORAK ATTORNEY Sept. 25, 1956 R. J. HORAK 2,754,060

PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Filed Sept. 23, 1952 8 Sheets-Sheet 2 f y g R RELAY p34 25 1 AND OPTICAL ELECTRONIC g; ELECTRICAL I COMPARTMENT COMPARTMENT COMPARTMENT m I5 I? 1 l- I 7%? 3 10 J E.M.F.PROPORTIONAL-M I g TO CONTRAST I E 5 l oui I I e: I

. LBLUE YELLOW T ME "TIME TIME mIIIIsecs.)

INVENTOR ROBERT J. HORAK ATTOR N E Y IGNAL POTENTIAL Sept. 25, 1956 J HQRAK PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Filed Sept. 25, 1952 8 Sheets-Sheet W K Y A F m A w mm W QNN I: m Q v B \Q @Q @N 0 W Q QF ,5 w R W J WQ 3 fink Y 3 B r v r my Allh F m @Q ww mum QM Sept. 25, 1956 R. J. HORAK PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Filed Sept. 23, 1952 s Shets-Sheet 4 n 1 J J 1 1 1 J u 1 PN-VQ. F E F a $2 r m .Q T 1% a fii m. A I m MN mwk m. j n w 9+ Wm H m a 1 J wsvwmq i i a m w 31 F I W -m.--m-i m v m Q E.. 9:. 5 m H Q 0 9509 g Q Q. 5...... N... T Q M Q Q g INVENTOR ROBERT J. HORA K hifgwgy/ ATTORNEY Sept. 25, 1956 R. .1. HORAK 2,764,060

PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Filed Sept. 23, 1952 8 Sheets-Sheet 5 IN VENTOR ROBERT J. HORAK f 711 QQJ Q DIRE CTION 0? WW} 169 LIG HT TRAVEL ATTORNEY Sept. 25, 1956 R. J. HORAK 2,764,060

PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Filed Sept. 2:5, 1952 I FOOQITCH IO 2 LOCK RELAY K 3 SHUTTER MOTOR M 4 PAPER TRAVERSE SOLL 5 PAPER TRAVERSE 6 LAMP RELAY K C 7 SCAN RELAY K BCONTRAST RELAY K STOTAL TIME RELAY K IO GUARD REL'AY K ll OPAQUE EXPOSURE BLUE EXPOSURE l3 YELLOW EXPOSURE REPRESENTATIVE DURA LEGEND III OPEN 0k NOT OPERA rnva l/V PROCESS OF OPENING OF CLOSING I c1. osso 0R OPERATING 8 Sheets-Sheet 6 LOW STOP OFF OFF START SCAN BLUE OR TIME FILM NEGATIVE EXPOSURE STATION -up FILM LIGHT SOURCE NEGATIVE AND FILTERS SUPPLY F b N /f\ PREVIEWER DODG'NG DUB i) STAT'ON B CONTROLS BA: 0 O

B B I mfl l l g ffi) &9 (D EXPOSURE STATION PHOTO FINISH- J 2 4 I ING PAPER Wyn El I I TAKE-UP Z v 4 0 PHOTOFINISHTNG 1 E2 3 4 1 PAPER SUPPLY 1 II I U [I h I I i INVENTOR ROBERT J. HORAK BY 777FOB -QI7/ Sept. 25, 1956 R. J. HORAK 2,754,060

PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Filed Sept. 23, 1952 8 Sheets-Sheet 7 LIGHT SOURCE YELLOW I DIRECTION OF Z 6 RELATIVE FREEDOM OPAQUE DIRECTION OF Z17 RELATIVE FREEDOM III" IIIIIIIIIIII J'III'II'I' LIGHT SOURCE FILM 8 PAPER Z20 ZZZ INV ENTOR ROBERT J. HORAK ATTORNEY 25, 1955 R. J. HORAK PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Filed Sept. 23, 1952 8 Sheets-Sheet 8 INVENTOR- ROBERT J. HORAK 7} 7, 5 OM17, v

ATTORNEY United States Patent Office 2,764,060 Patented Sept. 25, 1956 2,764,060 PHOTOGRAPHIC PRINTING EXPOSURE CONTROL Robert J. Horak, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application September 23, 1952, Serial No. 311,062

Claims. (CI. 88-24) This invention relates to improvements in photographic printing exposure control, and particularly to. the control of photographic printing exposure in accordance with the existing photographic density and photographic contrast of the developed negative film which it is desired to print.

It has been found that, in order to obtain pleasing contrast effects in positive photographic prints, the photographic printing exposure must be carried out with due regard for both the density and contrast characteristics of the negative film processed. One way in which the operator is afforded a broad range of selection of print contrast effects is through the use of variable contrast photofinishing paper, one type of which comprises that disclosed in U. S. Patents 2,202,026. and 2,280,300.

Variable contrast photofinishing paper was devised for the purpose of permitting the printing of developed negative films with preselected control of the contrast be tween light and dark portions of the negative ranging from very soft to very hard, depending on the effect desired. In order to obtain like results with photofinishing paper of one inherent. contrast characteristic, it is necessary to employ the particular grade. of paper which will provide thedesired contrast, which necessitates maintaining a supply of various grades of paper on hand and the placement of the appropriate paper in the printing apparatus. This is objectionable from the standpoint of expense, and. the time and labor involved.

The exposure of variable contrast photofinishing paper is accomplished by selecting a quality of light adapted to provide the desired contrast for the negative processed, very soft contrast being obtained with yellow light while very hard contrast is obtained with blue light. Since, normally, a degree of contrast is desired somewhere between these two extremes, it is preferred to expose the paper to light of each of these colors in the proper proportion, the ratio of the durations of the blue or yellow to the total illuminations then determining the contrast of the resulting print, while the total time of exposure is chosen in. conformity with the density of the negative. It has hitherto been the practice to obtain the information upon which the exposure is predicated as the subjective opinion of the operator, sometimesreached with the assistance of contrast meters, and then make the exposure by manual adjustment and control. This has been disadvantageous for a number of reasons, in that the judgment of individual operators, or even the same operator at different. times, has proved unreliable, trained negative and a second signal is also derived from the photoelectric current which is a function of the photographic contrast of the negative. Where the printing is conducted on variable contrast photofinishing paper, the first signal is utilized for the control of total exposure time, while the second signal is utilized for the control of the color of the exposure light, i. e. the quality of the light, as this term is hereinafter used in this description.

This invention is adapted for use in conjunction with conventional high-speed printing equipment and the apparatus for effecting the control conveniently comprises a self-contained unit which may be attached to commercially available printing machines as a substitute for, or at least a complement of, any controls originally incorporated therein. As will become apparent from the following description, the control achieved is fully automatic or semi-automatic, depending on the design of the printing machine per se, which is a highly advantageous feature of this invention.

In general, the apparatus of this invention comprises an electro-optical scanning system having a photoelectric detector which generates a pulsating E. M. F. in response to the light transmitted by the negative when a representative area of the negative is scanned by a Nipkow disk or like device. This E. M. F. is transmitted to two parallel connected electronic circuits, one of which indicates (or controls) the duration of the total light exposure as a function of the density of the negative, while the other indicates (or controls) the light quality as a function of the contrast of the negative. Two embodiments of this invention are hereinafter described in detail, the first being an adaptation for a conventional projection type photographic printer wherein the control achieved is fully automatic, and the second being an adaptation for a conventional contact type printer wherein the control. achieved is semi-automatic, in that the two signals on which exposure is predicated are indicated to the operator who thereafter manipulates the controls to effect the exposure accordingly.

A primary object of this invention is to provide an exposure control apparatus for photographic printers operating on variable. contrast photofinishing paper whichis automatic, or semi-automatic in operation.

Another object of this invention is to provide an exposure control apparatus for photographic printers operating on variable contrast photofinishing paper whichproduces prints of improvedquality, thereby reducing the number. of re-printings required.

Another object of this invention is to provide an ex:--

posure control apparatus. for photographic printers. operating on variable contrast photofinishing paper which has. an increased production rate.

Other objects of this invention include the provision.

of an exposure control apparatus for photographic printers operating on variable contrast photofinishing paper which is low in first cost. and maintenance, compact in arrangement, and adapted for attachment to a wide variety of designs of conventional. printing equipment.

The manner in which these and other objects of this invention are attained is disclosed in the detailed description illustrated by the following drawings, in which:

Fig. l is aschematic representation in side elevation of a conventional design of a projection type printer provided with one embodiment of exposure control apparatus constructed according to this invention,

Fig. 2 is a top plan view of a scanning disk,

Fig. 3 is a top' plan view of one embodimentof" exposure regulator disk,

Fig. 4 is a side elevation view of oneembodiment of scanning mask employed in-conjunction with the apparatus of Fig. 1, two adjacent scanning light spots and the courses described'during scanning being shown in broken line representation,

Fig. is a side elevation view of the housing of the exposure control apparatus of Fig. 1, showing the manual controls for calibration setting,

Fig. 6 is a sectional view of the apparatus of Fig. 5 taken along the longitudinal axis showing, partially schematically, the relative dispositions of the optical components of the apparatus,

Fig. 7 is a sectional view taken along line 7-7 of Fig. 5 in which all components mounted within the housing except the exposure regulator and the scanning disk are omitted,

Fig. 8 is an oscillogram showing typical E. M. F. traces secured in the course of scanning a negative with the apparatus of this invention,

Fig. 9 is a diagrammatic representation of the electronic exposure control circuit for the apparatus of Fig. 1,

Fig. 9A is a diagrammatic representation of the relay control circuit showing the connections to the lamp and motor constituting, with the electronic circuit of Fig. 9, part of the apparatus of Fig. 1,

Fig. 10 is a diagrammatic representation of the time relationship of the control e. m. f. for the regulation of exposure time and exposure light quality for a typical film negative requiring 50% blue light and 50% yellow light exposures,

Fig. 11 is a partially diagrammatic top plan representation of one embodiment of exposure regulator assembly according to this invention for use with the apparatus of Fig. 1,

Fig. 12 is a partially diagrammatic perspective representation of a focal plane type of light regulator assembly particularly adapted to high speed production printing for use with the apparatus of Fig. 1,

Fig. 13 is a schematic operating sequence diagram showing the interrelationship of the operation of the printer of Fig. 1 with the controls provided by this invention,

Fig. 14 is a schematic representation in side elevation of a conventional design of contact type printer provided with one embodiment of exposure control apparatus according to this invention,

Fig. 15 is a longitudinal sectional view of the housing of the scanning apparatus of Fig. 14,

Fig. 16 is a perspective view of the striated light filter in assembled relationship to the striated light regulator and light proportioning means of the apparatus of Fig. 14,

Fig. 17 is a longitudinal sectional view of the exposure light housing of the apparatus of Fig. 14, and

Fig. 18 is a diagrammatic representation of the electronic and associated electrical circuit of the apparatus of Fig. 14.

Referring to Fig. 1, one design of commercially available projection type photographic printer is shown schematically in which is embodied a light source 11 and associated optical elements including mirror 3 mounted in housing 2, a negative film holder 4, projection lens 5, photofinishing paper holder 6, guide rollers 7 and motordriven photofinishing paper storage rolls 8. All electrical equipment for this printer is contained within housing 9 and exposure is initiated by a foot treadle switch 10. A suitable electrical control (not shown) is provided for photoelectrically aided manual setting by the operator to obtain an exposure which the operator estimates will produce a satisfactory print, whereupon depression of switch 10 turns on the lamp 11 and subjects the negative film to light exposure for a corresponding period, after which a switch in the lamp circuit is opened automatically. A motor, not shown, then advances the paper a distance such that the exposed paper is moved towards the exposed paper storage roll and an unexposed length is presented for the next exposure cycle. It will be understood that printers of the design described can be employed for the printing of either single contrast photofinishing paper or variable contrast photofinishing paper;

however, this invention is concerned with the printing of variable contrast paper exclusively.

For the projection type printer the apparatus of this invention comprises an optical system for obtaining a light image of the negative processed, a scanner for obtaining the characteristics of the negative from the light image, an exposure regulator for controlling the quality of the printing light and the duration of exposure, and an electronic circuit for originating signals representative of the characteristics of the negative and for controlling the exposure regulator in accordance therewith. The principal operations performed by the electronic circuit are represented functionally in Fig. 1 by block representation, all electrical apparatus within the broken line enclosure being supplied with power from the power supply auxiliary 16 which is not part of this invention, nor part of the photographic printer, but is provided solely for interlocking the electrical apparatus of this invention with the electrical circuit built into the printer by the manufacturer. Power supply auxiliary 16 also furnishes power to the sequence control relays which initiate and limit the duration of exposure to the different colored light illuminations as well as implement the operation of the electronic circuit.

As shown in Figs. 1 and 6, the optical elements of the projection printer embodiment of this invention comprise beam splitter 20, which may be a type -720 Dichroic Filter manufactured by the Liberty Mirror Co., focusing lenses 21 and 22, and mirror 23. All of the optical elements are mounted in any convenient manner within the middle compartment of light-proof housing 24, which may be attached to the printer by suitable screws or other means, not shown, so that beam splitter 20 is interposed in the exposure light path between the film negative and the photofinishing paper. Exposure regulator disk 25, actuated by solenoid 140, is also mounted within housing 24 adjacent the top wall and athwart the exposure light path, and the scanning element 27, which in the illustrated embodiment is represented as a Nipkow disk driven by motor 28, is mounted inside the housing adjacent the bottom wall and athwart the image light path. The bottom wall of housing 24 is provided with an aperture in the image light path within which is mounted photoelectric detector 29, which may be an R. C. A. Type 929 phototube, and mask 30 is attached to the inside wall in optical alignment with scanning element 27 and detector 29. Compact arrangement of the components of the apparatus is secured by providing housing 24 with longitudinal partitions 34, such as those shown in Fig. 7, and assembling the optical apparatus within the middle compartment so formed, reserving one of the outside compartments for the electronic equipment and the other for the relays and other electrical auxiliaries. Electrical connection of the control apparatus with the printer is made by disconnecting the cable running from foot treadle switch 10 to the printing machine electrical equipment indicated at 9 and connecting it with the sequence control relays, as indicated in Fig. 1, the new relays being substituted as an assembly for any originally in the printer.

A suitable design of scanning element is shown in Fig. 2 and comprises a Nipkow disk provided with twelve circular light ports 37 spaced in a spiral pattern at equiangular locations referred to the center of rotation. Adjacent light ports are preferably disposed tangent one to the other, measured in a radial direction, so that the image area scanned, as represented by the broken line trace paths illustrated in Fig. 4, is substantially completely covered. The proportionate dimensions of the light-transmitting aperture 38 of mask 30 with respect to the negative image are not critical, since experience has shown that the primary features of interest in the usually-encountered negative are encompassed within the central 60% of area; however, it is preferred to employ an aperture of sufficient size to permit scanning about 90% of the area of the negative image. It is advantageous with this invention to utilize a continuous type signal, therefore adjacent ports 37 are spaced apart angularly a sufficient distance so that the advancing port of a pair just commences to clear aperture 38 as the following port reaches the aperture. In practice, a scanning resolution of about A" referred to the negative processed has proved entirely satisfactory, and an adequate scanning speed is obtained by the employment of a driving motor 28 having a speed of about 1600 R. P. M. In a typical apparatus constructed according to this invention, wherein negative prints approximately 3" x 3" in size were required, an optical system was utilized which gave a light image of approximately 4" x area, the scanning being accomplished with a 7" diam. disk provided with 12 holes approximately in diameter, the sixth hole counted circumferentially from the hole nearest the center of the disk being disposed 3%" from the center.

Referring to Fig. 3, one embodiment of exposure regulator comprises a metal disk 25 provided with light filters and a light-opaque expanse each of sufficient size to intercept the entire cross-section of the exposure light beam at the point where disk 25 is interposed thereacross. The blue filter may comprise a type OP-1O and the yellow filter a type OP-3, both marketed by the Optical Plastics Company. As will be noted from the drawing, the individual filters and the light-blocking area of the regulator disk are preferably proportioned so that each occupies about 120 of the circumferential expanse intercepting the exposure light beam, under which conditions equal rotations of disk 25 are required to position any one of the regulating elements in the exposure light path.

Photoelectric detector 29 is connected with the electronic circuit represented in Fig. 9 by leads 43 and 44-, the latter being provided with a tap connected to the left-hand grid of double triode cathode follower tube 45, which functions as an impedance converter. Tube 45 constitutes merely a design convenience in that the lefthand triode section reduces signal attenuation caused by long leads while the right-hand section compensates operation of the left-hand section. The voltage response of detector 29 is a linear function of the intensity of the light impinging thereon, and control of exposure can conveniently be based on the maximum negative transmission sensed, and thus the maximum potential generated by detector 29, and the diiference between the logarithms of the maximum and minimum transmissions of the negative, which is the same as the difference between the maximum and minimum densities. If desired, the former control characteristic may comprise the average transmission of the negative, instead of the maximum transmission, or some other factor detected by the scanning process.

Referring to Fig. 8, an oscillogram of two typical light scans across a negative image is represented, the instantaneous values of potential generated by detector 29 being plotted on the ordinate scale while time is plotted as the abscissa. The maximum potential achieved during a complete scanning cycle, which in the case of the scanning disk of Fig. 2 consists of twelve successive light traces corresponding to each of the light ports 37, constitutes the value of maximum transmission which, for purposes of explanation, may be assumed to be signal n of Fig. 8, while the signal representing the minimum transmission may be similarly assumed to be the value t. It will be understood that the scanning operation is continuous and that detector 29 therefore generates a pulsating signal of the type represented in Fig. 8 during the entire time that a light image is reflected from mirror 23; however, the subsequent electronic circuit requires only about five or six complete scanning cycles to effect control of exposure according to this invention, and the time required for acquisition of the information on 6 which the control is based is therefore very brief and is, in fact, only of the order of about 150v milliseconds or less.

The power supply for the apparatus of this invention is drawn from v. A. C. lines L1 and L2 connected through a magnetic circuit breaker 50 (Figs. 1 and 9) to power supply unit 16. Since it is desirable to obtain supply currents of several different voltages, 16 is provided with a multiple output self-regulating transformer 51, which may be a Sola Type Catalog No. 7105, a portion of the input being delivered as 6.3 v. A. C. current to leads La and L4 which supply current to all of the heater filaments of the electronic tubes hereinafter described through connections not shown, while the remainder passes to full-wave rectifier tube 52 which delivers 255 v. D. C. to lead L5 and v. D. C. to Le, L7 being grounded. The output conductors of rectifier 52 are provided with choke coil 53 and capacitors 54, which together constitute an A. C. removal filter, and also with voltage stabilizers 55 which each have associated R-C networks indicated generally at 56. Another pair of leads La and L9 is connected across L1 and L2 and furnish 115 v. A. C. current to leads L10 and L11, for powering through connections not shown lamp 11, the photofinishing paper traverse motor, scanner motor 28, and miscellaneous relays to be described, while also supplying full-wave selenium bridge rectifier 57 with current for conversion to 100 v. D. C. delivered to output leads L12 and L13, which comprise the power source for driving exposure regulator disk 25 as well as some of the D. C. relays hereinafter described. Pilot light 58 with its associated resistor is connected across leads La and L9 to furnish an indication of power delivery to 16 and thus to the entire apparatus.

L14 leading from the terminal connection of L6 supplies 150 v. D. C. power to tube 45 and, through the protective voltage divider indicated generally at 64, to photoelectric detector 29. L15 leading from the terminal connection of L5 supplies 255 v. D. C. power to all of the other electronic tubes of the exposure control apparatus of this invention.

The cathode of detector 29 is connected to ground through resistor 65, the grid of the left-hand section of tube 45 being connected in circuit as a cathode follower at point 66, which has a potential directly proportional to the light impinging on the photosensitive area of 29. Cathode follower 45 may be an R. C. A. Type 12AT7 tube having the grid of its right-hand section grounded, so that this section counterbalances the inherent increase in potential generated by the left-hand section at zero signal and thus compensates for error from this source. The reference potential level established by the righthand section of 45 is impressed on each of the parallelconnected electronic circuits hereinbefore referred to, the connection to the total light exposure control circuit being through lead L10 while the connection to the light quality control circuit is through L17. The cathode follower signal from the left-hand section of 45 is introduced into the total light exposure control circuit and the light quality control circuit through leads L18 and L19, respectively.

The characteristic of a developed photographic negative known as contrast constitutes the difference between the logarithms of the maximum and minimum light transmission therethrough which, as previously stated, is the same as the difference between the maximum and minimum densities. In order to make the most generally satisfactory prints, experience has shown that the logarithmic exposure scale of the paper should be made to equal the density range of the negative. The nominal logarithmic exposure scale for variable contrast photofinishing paper is 1 to 2, although these limits may vary to some degree between different rolls and provision must be made in apparatus of the type comprising this '7 invention to permit compensating scale corrections for attainment of the highest possible quality printing.

In general, based on the nominal exposure scale for the paper hereinabove referred to, where the negative to be printed possesses the minimum contrast denoted by the value 1 the quality of the printing light should be all blue, whereas a maximum contrast negative of 2 should be printed with all yellow light. Negatives having contrasts falling between the limits 1 to 2 are desirably printed with light qualities comprising optimum proportions of both blue and yellow and the exposure may be time-sequential, with an individual light of one color followed by an individual light of the other, or simultaneous, in which case the lights of the two colors may be suitably blended together with proper aperture control, both of these types of regulation being described in detail herein. It might be further mentioned that the photographic speeds of variable contrast photofinishing paper under blue and yellow illuminations are not always precisely equal, as indicated by the slopes of the lines of a representative time-sequential exposure (Fig. for a negative requiring exposure under 50% blue light and 50% yellow light, and suitable controls have been incorporated in the apparatus of this invention to permit compensation for this variable.

From the foregoing it will be understood that highest quality printing of variable contrast photofinishing paper can only be achieved by regulation of both the duration of the total exposure to light and also the quality of the light and that, to a degree at least, the absolute valve of the former will depend upon the latter. According to this invention this regulation is achieved by controlling the total time of exposure and, independently, the duration of blue light exposure, whereupon the duration of yellow light exposure is constrained to proper limits indirectly. The maximum total time required for the printing of a single negative is approximately 2 seconds, the paper indexing, lamp 11 heat up and the signal deriving scanning occurring during the first second while the actual exposure follows during the remaining time.

The exposure control circuit, which effectively measures the maximum negative transmission and computes the total exposure time, the latter operation with the assistance of some information derived from the light quality control circuit, will first be described with particular reference to Figs. 9 and 9A. In the interest of clarity of representation the several relays and other components of Fig. 9A have been set off in vertical blocks defined by irregularly broken lines, so that ready identification of each element may be had by recitation of the appropriate block heading followed by the letter identifying the particular set of relay contacts involved, all switches, unless otherwise denoted, being simply referred to by the letter S with a characteristic numeral subscript for each.

The signal E. M. F. from the left-hand triode of cathode follower 45 passes through L18, which is provided with a load resistor 71 connected to ground, to peak detecting germanium diode 72, and thence to normally open relay contacts KZD which, upon closing, complete the circuit to capacitor 73. Double triode tube 74, which may be an R. C. A. type 12AX7, the left-hand section of which constitutes part of the exposure control circuit and the right-hand section of which is part of the light quality control circuit, has its left-hand cathode connected to the other side of 73. The assembly comprising the left-hand triode section of 7d, capacitor 73, adjustable resistance controls C1 and C2, capacitor 75 and resistor 76 together with the circuit conductors shown in Fig. 9 constitute a so-called bootstrap sweep circuit. It will be understood that, in the circuit shown utilizing cathode follower 45, capacitor 75 is charged from the right-hand triode section of 45 and thus extends the compensation thereof. If cathode follower 45 is dispensed with, capacitor 75 would be retained but would, in that case, be shunted to ground through relay contacts K2E hereinafter described. The adjustable resistor manipulated by C2 is shunted through normally closed relay contacts KlA by L20.

The cathode of the left-hand section of tube 74 is provided with a load resistor 76 and is connected through another resistor, 77, with the left-hand grid of double triode amplitude comparator tube 78, which may be an R. C. A. 12AT7 type with its cathodes connected to ground through resistor 79. The right-hand grid of 78 is connected through fixed resistor 89 and adjustable resistor 81 to +255 v. D. C. (L15), and also through 81 to ground. The left-hand plate of 73 is in circuit with the coil 82 of the relay Ks, which may be a Western Electric Relay Type No. 276-B. Relay Ks, has its movable switch arm connected to L21, and thus to normally open relay contacts KsA, contact Kea connected to L22, and thus to normally closed relay contacts K2A and nor mally open relay contacts K2B,, contact Ksb connected to L23, and thus to normally closed relay contacts K38 and to the powering lead L25 for the positioning of the opaque portion of regulator disk 25 in the exposure light path, and contact K60 connected to L24, and thus to relay coil K4 provided with time delay capacitor KiCx.

The reference E. M. F. signal emanating from the right-hand triode section of 45, the cathode of which is grounded through load resistor 70, is introduced into the total exposure control circuit through L10, as hereinbefore described, which is connected across normally open relay switch contacts K2E to one side of capacitor 75, the other side of which is grounded. A clearing conductor for grounding capacitor 73 is provided by L26 connected across normally closed relay contacts KaF.

Finally, the total exposure control circuit is provided with a non-typical negative alarm auxiliary comprisng double triode automatic processing limit signaling tube 90, which may be an R. C. A. Type l2AT7, with the lefthand grid connected to the output side of diode 72 through resistor 91 and to ground through resistor 92, of the same resistance value as 91, and the right-hand grid connected to the input side of 72 between resistor 93 and grounded capacitor 94, which together constitute a low pass filter. The cathodes of both sections of tube are connected together and grounded through variable resistor 95, and both plates are provided with in dividual resistors 96 in circuit with common lead L22 which is connected across normally open relay contacts K2C through L23 running to the terminal connector of L5. The alarm signal device for this auxiliary circuit comprises buzzer 97 shunted across the plates of tube 9-1.

Turning now to the light quality control circuit, the signal E. M. F. derived from the left-hand triode of is supplied through L19 to resistor 101, which has a resistance value of the order of 10 megohms, and thence to the right-hand grid of tube 74, the left-hand triode section of 74 being conveniently utilized in the total exposure control circuit as has already been mentioned. A positive feed-back connection L29, including capacitor 102, is provided between the right-hand grid of 74 and adjustable resistor 103, the purpose of which will be hereinafter described in greater detail in the explanation of the operation of this invention. The right-hand triode secton of 74 and the entirety of tube 194, which may be an R. C. A. 12AT7 type, function together as a logarithmic amplifier, the plates of which, in succession from left to right as seen in Fig. 9, are powered from L1 through resistor 105, resistor 106 and conductor 107, respectively. The right-hand plate of 74 is coupled to the left-hand grid of 104 through L30 including capacitor 103, and the left-hand plate of 104 is similarly coupled to the right-hand grid of 184 through L31 provided with capacitor 109. Both grids of tube 194 are in circuit with their associated cathodes, the left-hand grid through resistor 110 and the right-hand grid through resistor 111, while 112 and 113 constitute load resistors for the lefthand cathode and 114 and 103, hereinabove described, render a similar service to the right-hand cathode.

The output signal from tube 104 is coupled through L32, and capacitor 118, to peak-to-peak detector tube 119 which may be of the R. C. A. 12AT7 type, by connection with the left-hand plate and grid and the righthand cathode thereof. The left-hand cathode of 119 is connected to one side of logarithm light transmission dilference capacitor 120 and also to a point between the resistors of controls C and C4 by L33, and the righthand plate and grid are connected to the opposite side of 120 and also to the opposite side of the resistors of C3. L45 establishes a clearing connection for capacitor 121 through normally closed relay contacts KsG. C3 constitutes an overall gain control for the entire light quality control circuit and comprises a fixed resistor 121 in series with a variable resistor 122, the adjustable tap of which is connected to the right-hand grid of amplitude comparator tube 128, which may again be of the R. C. A. 12AT7 type. C4 is a zero suppressor comprising a variable resistor 123 connected on one side to power lead L15 through fixed resistor 124 and on the other to ground. The common conductor between C3 and C4 is connected to ground through capacitor 125.

Both the right and left-hand plates of 128 receive power from L15 through individual connections therewith, the left-hand plate being in circuit with coil 129 of relay K7, which may be of the same type as Ks except that only two of the contacts provided in the device are utilized. The cathodes are connected together and grounded through load resistor 130, and the left-hand grid interlocks the exposure control and light quality control circuits through L31 provided with resistor 131 connected between the left-hand cathode of 74 and resistor 77. Contact Km is connected to the blue exposure filter actuating lead L35, while contact Krb is connected to the yellow exposure filter actuating lead L36, L3 leading to L13 through the coil of K1. The switch arm of K7 is connected to normally closed relay contacts K2A through L38.

Referring to Fig. 9A, additional circuit elements, all of which are shown in the position assumed immediately before the beginning of a scanning cycle, comprise shutter motor M1, which is a part of the printer per se and conventionally may be a capacitor-run induction motor, and paper traverse actuating solenoid coil L, also part of the printer. A number of switche are also necessary, some of which may constitute standard components of the printer, depending upon its design, but others of which must be introduced to complete the circuit of this invention. These comprise mechanically actuated paper traverse-operated switch S1 in circuit with relay coil K2 between power supply lines L10 and L11, foot treadle switch S10 in circuit with relay coil K5, normally closed relay contacts K4B and contact a of shutter motor switch S6, and the switches S2, S6, and S8 which are actuated by M1 through mechanical connections denoted by broken lines 135, 136 and 137, respectively. For the sake of brevity, any relay contact pairs or other items not hereinabove specifically mentioned will be described and their purposes made clear in the description of the operation of this device hereinafter set forth.

Referring to Fig. 11, one embodiment of exposure regulator drive, indicated generally at 140, Figs. 1, 5, 6 and 7, comprises a solenoid 141 for driving disk 25, the sequence of operation being in a clock-wise sense in response to signals received from the circuit of Figs. '9 and 9A through leads L25 (opaque), L35 (blue) and L36 (yellow). Disk 25 is shown in Fig. 11 in the position assumed immediately before the beginning of an exposure cycle, the boundaries of the exposure light beam being denoted in broken line representation at 278 with all light blocked off from passage to the photofinishing paper by the opaque portion of the disk. Disk 25 is keyed to shaft 142, suitably journaled in a bearing not shown, and is 10 provided with a drive plate 143 fixed thereto having drive pins 144 and peripheral drive stop serrations 145 for each of the three exposure conditions opaque, blue and yellow.

The drive mechanism for disk 25 comprises the mechanical linkage 146147-148, supported on pivot 149 carried by the frame of the housing of 140, link 146 being pinned to link 147 on one side of 149 with its opposite end in a position to abut the drive pin 144 of the preceding exposure station and link 148 being pinned to 147 on the opposite side of pivot 149. The free end of link 146 is machined with a concave profile 162 and when in abutment with pins 144 engages a considerable portion of the periphery of the pins. Link 146 may be guided in suitable ways, not shown, attached to the frame or may merely be rigidly pinned to 147 so that it is fixed with respect thereto. Stop pin 150, fixed to link 147, is provided for engagement with stop serrations 145, and link 147 is continually urged in the direction of the axis of shaft 142 by biasing spring 151 secured to the frame at 152. Link 148 constitutes the movable plunger of solenoid 141 and is displaced outwardly, i. e. in a direction away from the axis of shaft 142, when 141 is energized, therebydisengaging pin from an adjacent serration 145 and freeing disk 25 for rotation under the inward movement of the free end of link 146 in abutment at 162 with an adjacent drive pin 144.

The sequential movement of disk 25 is effected through the agency of homing wafer 155, which may be of the conventional design marketed by the G. H. Leland Co. in its Ledex Circuit Selector, fixed to drive plate 143 so as to rotate in accompaniment therewith, and provided with a perpetual closed circuit contact 156 and three make-and-break contacts as follows: 157 for opaque exposure, 158 for blue exposure and 159 for yellow exposure. Homing wafer is cut away along the periphery at point 154 to break the electrical circuit with any one of the contacts 157, 158 and 159 when disposed adjacent thereto. Contact 156 is in circuit through L39 with L40 of solenoid 141, L41 of which is connected to L13 (Fig. 9A), and an arc-suppressor comprising resistor 160, capacitor 161, and L42 connecting to L12 (Fig. 9A) is also provided.

The operation of the device of this invention will next be described with particular reference to Fig. 13 which depicts the time sequence in which the several individual steps occur, it being understood that the relative durations of the steps are merely representative in the drawing, slnce these durations depend on the density and contrast characteristics of each particular negative processed.

At the initiation of an operating cycle the opaque area of exposure regulator disk 25 is interposed between the negative to be printed and the photofinishing paper, so that no exposure can occur until the scanning is completed and the corresponding scanning signals for the control of the exposure are derived.

The entire operation of the printer and the apparatus of this invention is initiated by the operator on depression of foot treadle switch 10, Figs. 1 and 9A, which thereupon closes the circuit between power lines L111 and L11 through normally closed relay contacts K413 and relay coil K5, at the same time energizing shutter motor M1 through closed contacts Sea. K5A is a normally open locking contact, which closes under the influence of K5 and thus maintains closed the powering circuit to K5 and to M1 until this circuit is independently opened by means hereinafter described. Motor M1 drives the shutter (not shown), provided as a standard component of the printer, from fully closed position to fully open position and, at the same time, actuates switch S through mechanical connection 136, from closure of contacts Sea to closure of contacts Seb, thus stopping further rotation of M1. Simultaneously, S8 is momentarily closed by M1 through mechanical connection 137, which energizes solenoid L, unlatching the paper traverse (not shown) and causing the paper traverse motor to index a fresh length of photofinishing paper into position to be printed. At the instant of closure of contacts Ssb, M1 also closes S2 through mechanical connection 135, which is preferably a cam, thus completing the circuit through normally closed contacts K4A to relay coil K3 across power lines L and L11, thereby effecting completion of the power circuit to the printer lamp 11 by closure of normally open contacts KsC. This entire sequence is represented by the first six blocks of Fig. 13, the total time consumed for this part of the cycle being of the order of 0.2 sec.

Whenever the photofinishing paper is traversing, S1 is closed by mechanical connection 138 running to the paper traversing mechanism and, accordingly, relay K2 is energized at these times. Were it not for this fact, energization of relay K3 would operate to interrupt 100 v. D. C. power delivery to L25 (opaque) upon the opening of normally closed contacts KaB, the continued connection during paper traverse being made through KsA, L21, K Kea, L22, and K2B. Therefore, the opaque portion of disk 25 remains interposed across the light path as indicated by block 11, Fig. 13.

K2 is denoted the Scan Relay, line 7, Fig. 13, for the reason that it initiates scanning of the negative by the closure of normally open relay contacts K2D across which L12 is connected and completes the circuit from germanium diode 72 to the balance of the total exposure control circuit. K2 at the same time closes normally open contacts IQE, completing the circuit through L16 and permitting passage of the reference voltage signal from the right-hand triode of 45 to the total exposure control circuit. Lamp 11, while not completely heated up, is by this time sufficiently incandescent to permit successful scanning of the negative, it being understood that full incandescence is attained substantially before the completion of the scanning period.

As previously mentioned, scanning occurs during the entire operating cycle and, indeed, is continuous, since the power leads of scanner motor 28 are in circuit with power lines L10 and L11 by connections not shown, and thus motor 28 is powered at all times during which circuit breaker 50 is closed. As hereinabove stated, how ever, only a very brief portion of the cycle time is required for the derivation of the information upon which exposure is predicated, wherefor the balance of the scanning is not utilized and therefore is not necessary to the operation of this invention.

As shown in Fig. 8, detector 29, during the scanning of a typical negative, generates a pulsating signal, the instantaneous value of which is a linear function of the intensity of the light impinging on 29 and, thus, of the maximum transmission of the negative in process. Re ferring to Fig. 9, this pulsating signal is transmitted by tube 45 and delivered from the left-hand triode section to the exposure control circuit, determinative of the total exposure time required, through L12, and to the light quality control circuit through L19. Simultaneously, the right-hand triode of tube 45 functions as a cathode follower, since no signal is impressed on the right-hand grid, and thus delivers a reference potential signal which is impressed on the exposure control circuit through L1 and on the light quality control circuit through L17.

Considering first the exposure control circuit, the signal first passes peak-detecting diode 72 and, relay contacts K2D being closed, charges capacitor 73 to a voltage corresponding to the maximum transmission of the negative plus the extra Zero signal voltage generated by the left hand triode section of tube 45. At the same time the right-hand triode section of 45, relay contacts K2E being closed, charges capacitor 75 to zero signal voltage level. Thus, after charge-up, it will be understood that the difference in potential between capacitors 73 and 75 is directly proportional to the maximum light transmission of the negative in process.

Control of total exposure time is achieved by the rate of potential rise of the left-hand grid of tube 74 under the influence of capacitor 73 after the inactivation of K2 and the return of contacts K2E to their normally open position. This rate of potential rise is directly propor tional to the maximum light transmission of the negative and inversely proportional to the R-C product of the combined resistances of C1 and C2 interposed in the circuit at the particular moment and of the capacitance of 75, but is largely independent of the capacitance of 73, since 73 has a larger capacitance value (1.0 mmf.) as compared to 75 (0.01 mmf.). The comparison E. M. F. plotted as the ordinate in Fig. 10 is the difference in potential between the two grids of each of the comparator tubes 78 and 128 before exposure starts and, in a typical case, has a finite value of 10 v., while the slopes of the blue time line and the yellow time line demonstrate that the rate of charge for the blue exposure is greater (and thus the blue exposure time is less) than for the yellow exposure. Also, it will be understood that the time contribution for blue is indirectly determinative of the total exposure, since the slope of the yellow line will, within the usual variations encountered in the manufacture of variable contrast paper, always be approximately the same, and thus the longer the blue exposure the shorter will be the yellow, the converse also being true. Under blue exposure the preset resistance of C1 alone is in circuit with capacitor 75, C2 being shunted by normally closed relay contacts KIA, which are open only during the yellow exposure period, and therefore the rate of potential rise of the left-hand grid of 74, is relatively rapid. Theoretically, with careful choice of circuit constants, photofinishing paper of invariable characteristics and absolutely stable operating conditions, control C1 would not be required; however, practically, it serves a useful purpose in permitting compensations in system speeds to correct for the effects of any dust or dirt which may enter housing 24, degradation of lamp 11 and the optical components during long term use, occasional variations in photofinishing paper characteristics, and the like. C1 preferably comprises a 10 megohm tapped resistor having a manual control dial mounted exteriorly of housing 24 in a location readily accessible to the operator, as shown in Fig. 5. C2 may likewise be a 10 megohm tapped resistor, provided with a control similar to C1 and located adjacent thereto. The initial calibration of C1 and C2 is here inafter described in detail together with controls Ca and C4, the latter two of which constitute part of the light quality control circuit.

During the scan cycle the potential across capacitor 73 immediately commences to rise and attains a constant level, which depends on the maximum light transmission of the negative, well before the end of the cycle, the potential across capacitor 75 remaining constant at the zero signal reference level. At the end of the scan cycle, which occurs when relay contacts K2D and K213 open as switch S1 opens a slight interval after paper traverse is completed, the potentials of capacitors 73 and 75, and of the left-hand cathode of 74 all rise at the same rate, the rise being started with 75 and imparted to 73 through the cathode follower action of the left-hand section of 74, which rise is dependent on the maximum light transmission of the negative in process, thus commencing the exposure cycle. This rise in potential starts the left-hand triode section of 74 functioning as a saw-tooth generator in accompaniment with the bootstrap sweep circuit of which it is part, and a characteristic signal is generated which passes through resistor 77 to the left-hand grid of amplitude comparator tube 78 for the computation of the total exposure period. The opening of relay K2 at the end of the scan cycle effects the shifting of exposure regulator disk 25 from opaque to a position where the blue filter is interposed across the light path, the circuit from L12 being via KsA (relay Ka being closed at this time), L21, Ksa, L22, K2A, L38, K'za, L35 and contact 158 to homing wafer 155. Since the relative positions of the makeand-break contacts are as shown in Fig. 11 at this part-i'c'ular instant, the circuit is then completed through 156, L39, L10, solenoid coil 141 and L41 to L13, and plunger 148 is displaced downwardly, momentarily releasing stop pin 150 and imparting a propulsive force through link 146 to the drive pin 144 against which the free end 162 abuts, thereby indexing disk 25 in a clockwise direction. When the cut-away portion 154 of wafer 155 comes into juxtaposition with contact 158, solenoid 141 is de-energized and stop pin 150 is biased inwardly by spring 151, positively locking disk 25 in position for the balance of the blue exposure.

The next step in operation, i. e. the yellow exposure period represented by block 13 of Fig. 13, necessitates consideration of the light quality control circuit, postponing explanation of the operation of the remainder of the total exposure circuit until later in this description.

As previously mentioned, the cathode follower signal from the left-hand section of tube 45 and the reference potential of the right-hand section of 45 are impressed on the light quality control circuit through L19 and L11, respectively, the former to the grid of the right-hand triode of 74 through resistor 101 and the latter to the corresponding cathode. The resistance of 101 is of the order of 10 megohms, whereupon it will be understood that the grid is driven by an appreciably higher potential source than the cathode and, as is well known to those skilled in the art, the potential difference between the grid and the cathode of the right-hand section of 74 thereby becomes logarithmically proportional to the instantaneous input signal, i. e. to the instantaneous light transmission of the negative in process. The resulting grid-cathode signal is amplified by the right-hand section of '74 and appears at its plate, which is coupled to the left-hand grid of 104 where a second stage amplification occurs. The signal appearing at the left-hand plate of 104 is coupled to the right-hand grid of 104, the right-hand triode of which constitutes a cathode follower, and is then impressed on the left hand plate and grid of peak-to-peak detector tube 119. Adjustable resistor 103, in circuit with the righthand cathode of 104, with tap connected through capacitor 102 and L29 to the right-hand grid of 74, is provided as a positive feed-back to prevent dissipation of the high frequency components of the scan signal.

It will be noted that the signal emanating from the right-hand section of 104 is coupled to both the left-hand grid and the right-hand cathode of tube 119, whereupon the minus D. C. potential appearing across capacitor 120 is proportional to the ditference between the logarithms of the maximum and minimum light transmissions of the negative, i. e. to the characteristic defined as contrast in the photographic art. This potential is impressed on the right-hand grid of amplitude comparator tube 128 through fixed resistor 121 and the variable resistance 122 controlled by the tap of Ca.

Overall gain control Cs, for which an operating dial is provided (refer Fig. permits selection of any desired output scale within a range of approximately Ol2.5 v. to match the upper end of the logarithmic exposure scale of variable contrast photofinishing paper, while zero suppressor C4 permits a corresponding match at the base end of the scale. With proper calibration settings, made as hereinafter described, a control signal derived from C3 and impressed on the right-hand grid of amplitude comparator tube 128 will effect operation of relay K7 and index regulator disk 25 from the blue exposure position to the yellow exposure position.

The two amplitude comparator tubes 78 and 128 function in an identical manner, but in response to different signals, wherefor the explanation of the operation of both will be treated together insofar as practicable. Tubes '78 and 128, and tube pairs 237-441 and 261262 hereinafter described for the contact printer embodiment of this invention, constitute means equating the potential level of the signals representative of the film negative density and contrast to 'a datum reference potential which may be 14 either round or some potential different than ground, depending on the design of the electronic circuits.

The signal impressed on the left-hand grids of both 78 and 128 is the same, resistances 77 and 131 being equal (preferably about 4.7 megohms), and the signal being derived in common from the left-hand cathode of tube 74. Beginning at the start of the exposure cycle, the potential of the left-hand grids of 78 and 128 commences to rise in accordance with the rise in potential of the lefthand cathode of 74. The right-hand triodes of tubes 78 and 128 are powered independently, the former directly from L15 and the latter from the tap of Ca. Accordingly, the flow of current in both right-hand triode sections begins to shift to the corresponding left-hand triode sections and, when the potentials of the left and right-hand grids in each tube are approximately equal, the design is such that the left-hand triode section of tube 128 energizes coil 129 of relay K7 and the left-hand triode section of tube 78 energizes coil 82 of relay Ks, operation being in the order recited.

Considering the operation of relay K7 first, the switch arm at this instant opens contact Km and closes contact K'zb, energizing relay K1 (through Law) which opens contacts K1A and interposes the preset resistance of C2 in the bootstrap sweep circuit, thus decreasing the rate of rise of potential of the left-hand section of 74 in a manner previously described. At the same time K7 interrupts the circuit to L35 and establishes in its stead the yellow circuit through L36. Current thereupon flows to contact 159 (Fig. 11), homing wafer 155, contact 156, L39, L40, solenoid coil 141 and L41, actuating the linkage 146- 147-148 and unlocking 150, all in the manner previously described for the opaque to blue index, thereby interposing the yellow filter in the exposure light path in place of the blue filter. Yellow exposure then ensues as indicated by block 13, Fig. 13.

During the entire yellow exposure the potential of the left-hand grid of amplitude comparator tube 78 continues to rise, but at a somewhat slower rate than during the blue exposure, due to the fact that the resistance of C2 is now in the grid circuit. Thus, the potential rises at a relative rate represented by the flatter slope of the yellow line (Fig. 10), rather than according to the steep slope of the blue exposure. The total exposure cycle is terminated when the potentials of the two grids of 78 are approximately equal, at which time the design is such that sufficient current flows through the left-hand triode section of 78 to energize coil 82 and operate relay K6.

The exposure cycle which has been hereinabove described may be represented mathematically by the expression:

Blue R.-hand g id l2BELeft-hand cathode 74 (of start) Total Rt.-hund grid 78 Left-hand cathode 74 (at start) wherein TB1ue=TiInC of blue exposure TTota1=Tlme of blue plus yellow exposures E=E. M. F.

Operation of relay K6 from contact Ksa to contacts Ksb and Ksc, which latter are closed simultaneously, interrupts the circuit from L12, through closed contacts KsA, L21, Ksa, L22, K2A, L38, Kqb to Lss (the yellow exposure actuation line), and completes it from Kab through L23 to L25 (the opaque exposure actuation line). Referring to Fig. 11, current now flow from L25 through contact 157, cut-away portion 154 being at this instant opposite 159, homing wafer 155 and contact 156, thence through L39, L40, solenoid coil 141 and L41, thereby actuating linkage 146-147-148 to retract stop pin and index disk 25 so as to move the opaque section across the exposure light beam in place of the yellow, this operation being indicated by the second shaded block of line 11, Fig. 13. At the same time guard relay K4 is momentarily energized upon the completion of the circuit through Ksc to L24 (line 10, Fig. 13), tie-energizing lamp relay K3, by opening contacts K4A, and restoring KaC to open position (line 6, Fig. 13). K4 also opens contacts K413, deenergizing K5 and thereby discontinuing the locking action of K5A (line 2, Fig. 13). The de-energization of relays K3 and K5 restores the powering circuit from L11 to shutter motor M1 through KsE, K5B and $517, which latter contacts are at this time closed, and motor M1 operates to close the printing machine shutter (line 3, Fig. 13) and to restore switches S2, S6 and S8 to their original open positions, i. e., to the positions represented in Fig. 9A, by virtue of mechanical actuation through 135, 136 and 137, respectively.

As hereinbefore mentioned, relay K4 de-energized K3, thus restoring contacts K3F to their normally closed position, which grounds capacitor 73 through L26 and thus clears the exposure control circuit of any residual charge, so that it is in condition for the next succeeding operating cycle. The de-energizing of K also causes normally closed contacts KsG to short circuit the plates of capacitor 120 through L45, thereby dissipating any residual charge thereon and restoring the light quality control circuit to its original condition for another operating cycle. These two circuit clearing actions immediately cause the potentials of the left-hand grids of amplitude comparator tubes 78 and 128 to drop, de-energizing total time relay Ks (line 9, Fig. 13) and contrast relay K7 (line 3, Fig. 13), respectively, and closing the circuit to normal position Kea for the former and to normal position Km for the latter.

Since Mr is a capacitor-controlled motor, a residual charge remains on capacitor M1C after closure of the printing machine shutter which, upon making of contact Sea, might tend to energize relay K5 by fiow of transient current through its coil, which would cause uncontrolled repetition of the entire operating cycle. As a safeguard against this contingency, the coil of guard relay K4 is shunted with capacitor Cx so that, even when the powering circuit to K4 is interrupted by the switching of K6 from position Ksc to Ken, K4 remains energized from its own R-C loop for a sufficient extra interval of time (line 10, Fig. 13) to maintain contacts K413 open until complete discharge of MIC occurs. When K4 opens, the entire control circuit is completely restored to the condition represented in Figs. 9 and 9A, and the operator can immediately commence the printing of another film negative by simply depressing foot treadle switch 10. It will be understood that during the course of an operating cycle any depression of treadle switch 10, either accidental or intentional, will have absolutely no effect until the cycle is completed and the circuit has automatically returned to its original state.

Turning now to the calibration of controls C1, C2, C3 and C4, the following procedure has proved entirely satisfactory. It is essential that all of the controls be calibrated together, and in the sequence hereinafter recited, or otherwise the adjustment of one will introduce error into the setting of one or more of the others.

C1 should be calibrated first, C2 being adjusted to zero, i. e. with its tap moved as far towards ground in the direction of capacitor 125 as possible, and C4 being adjusted in the opposite direction, i. e. with its tap farthest removed from ground, or in the direction of resistor 124, Fig. 9. These adjustments of C3 and C4 give an all blue light exposure. A step wedge negative is then placed in the negative holder and a sample print made and developed. The setting of C1 is then adjusted, on the basis of observations of a succession of developed sample prints, until a print is obtained which is just full black for the corresponding portion of the step wedge.

C2 is calibrated next, the setting of C3 being preserved unchanged from its setting during the calibration of C1 but the setting of C4 being altered by moving its tap to ground. C3 and C; under these settings give an all yellow light exposure. C2 is then calibrated, using the same step 16 wedge and the same procedure hereinabove described for the C1 calibration.

Finally, with both C1 and C2 at their proper calibration settings, C and C4 are calibrated together. For this calibration it is necessary to employ two negatives of contrast approximating within :0.1 unit the extremes of the logarithmic exposure scale for variable contrast photofinishing paper, i. e. 0.9l.1 for the base end of the scale and l.92.1 for the upper end of the scale. A succession of sample prints is made from each negative in turn, retaining the same settings for C3 and C4 and, when it is visually determined that an all blue exposure is obtained for the low contrast negative (O.91.l on the exposure scale) and an all yellow exposure is obtained for the high contrast negative (1.9 to 2.1 on the exposure scale), Cs and C4 will be properly calibrated.

In practice, the calibrations of C1 and C2 should be checked daily, or upon beginning use of a new roll of printing paper in the machine. Ordinarily, it has been found that it is not necessary to alter the setting-s of C3 and C4 so often, unless the recalibrations of C1 and C2 are widely different from their previous calibrations; however, it is preferred to check the calibrations of C3 and Ct at least once a week as a matter of routine. It will be understood that the manner of operation during calibration is exactly the same as it is during production printing.

The use of the non-typical negative alarm circuit comprising double triode tube 9i and its associated circuit in conjunction with this invention is optional, depending on the printing quality standards which the user desires to adhere to. Rather infrequently, a negative may be encountered wherein the peculiar subject matter represents a deviation from the norm in that the average light transmission thereof does not closely approximate half of the maximum light transmission. Such a negative represents a special case and therefore requires special printing procedures in order to achieve the highest possible quality. The purpose of the non-typical negative alarm auxiliary is to detect the presence of such non-typical negatives, so that the operator may be advised and may mark them for special consideration.

The function of tube is to equate a signal corresponding to half of the maximum light transmission of the negative against a signal corresponding to the average light transmission of the negative, and to actuate alarm buzzer 97 if a substantial difference exists between these two signals. The left-hand triode section of 90 draws its signal from the output side of diode 72 and, because resistance 91 is equal to resistance 92, the potential assumed by the left-hand grid will thus correspond to half that attributable to the maximum light transmitted by the negative. The low pass filter circuit supplying the signal from the input side of diode 72 to the right-hand grid is conventional and well know to those skilled in the art, and 1t inherently derives a control potential for the right-hand grid of tube 90 which is proportionate to the average light transmission of the negative. When a condition of unbalance occurs between the left and right-hand triodes of tube 90, current will flow across one triode section or the other, producing current flow through buzzer 97 and thereby giving audible Warning to the operator that a special negative is in the machine. The power for operating buzzer 97 is drawn through L27 across relay contacts K2C, which are closed during the scan cycle as hereinbefore described (refer also line 7, Fig. 13), which connect through L28 with power line L5 (Fig. 9).

This invention has been described with particular reference to the embodiment of exposure regulator shown in Fig. 11, which has the advantages of simplicity in construction and low space requirements. However, for high speed production printing the focal plane type of light regulator shown in Fig. 12 has certain advantages which make it preferred.

This regulator comprises an assembly of three separate shutters, two of which are semi-circularly shaped opaque shutters and the third of which is a complete circle, onehalf of which is a blue light filter and the other half a yellow light filter. All of the shutters are supported through individual slip clutches 167 from a common, vertically disposed, constant speed shaft 168 rotatably driven by a motor (not shown), Fig. 12 shows the relative orientation of all three shutters just prior to the beginning of an operating cycle, the limits of the exposure light beam being represented by broken line projection 169 and opaque shutter 170 being interposed across the light path in a position preventing exposure of the photofinishing paper. At this time opaque shutter 171 is disposed 180 degs. away from 170, so that it is completely out of the light path, and the two-color shutter 172 is oriented with its blue half intercepting the light. Each of the shutters is provided with peripheral locking recesses 173, and shutters 170 and 172 are in addition provided with peripheral stop hooks 174 and 175, respectively. Except during the operating cycle, each of the shutters is locked in the positions represented in Fig. 12 by the individual solenoids 176, 177 and 17 8, the retractable plungers of which engage with recesses 173.

Vertical rock shaft 179, disposed diametrically opposite solenoids 176, 177 and 178, and journaled in a bearing not shown, is provided to prevent the shutters from uncontrollably traversing the light path once they are released by the solenoids, stop arms 180 and 181, keyed to the shaft, being formed at the ends adjacent the shutters with hooks adapted to engage stop hooks 174 and 175, respectively. It will be understood that the inclinations of the mating surfaces of the hooks of the stop arms and the stop hooks are such that, on engagement, the application of force is in a substantially radial direction referred to rockshaft 179, so that no turning movement is applied to the latter by the shutters, the clutches 1670f which accordingly slip on drive shaft 168 and the corresponding shutters remaining stationary. Rockshaft 179 is further provided with a bellcrank 182, keyed thereto and disposed in the plane of shutter 171, the outer arm of which is biased by a weak spring 183 against pin 184 integral with the machine frame. The inner arm of bell-crank 182 is sufficiently long to abut the leading edge of shutter 171, so that, when the plunger of solenoid 177 is retracted and 171 is turned by the rotation of shaft 168 through the middle coupling slip clutch 167, rock-shaft 179 will be turned in a counter-clockwise direction as seen in Fig. 12, thereby releasing the engaged hook pairs 180174 and 181-175 and freeing shutters 170 and 172 for completion of their rotation to locking position adjacent solenoids 176 and 178, respectively.

The electrical circuit for the light regulator of Fig, 12 is similar to that of the regulator of Fig. 11, the blue powering line L3 leading to solenoid 176, the yellow powering line L36 leading to the solenoid 178 and the opaque powering line L25 leading to solenoid 177. The other connection to the solenoids is through a common lead L46, corresponding to L41 of Fig. 11, which is connected to l00 v. D. C. line L12 in the same manner as shown for L41 in Fig. 9A. Since it is desired to obtain only a momentary retraction of the plunger of solenoid 177 associated with opaque shutter 171, a differentiating network comprising capacitor 185, in series with L25, and a resistor 186, in parallel therewith, is provided, the effect of which is to produce pulsed operation of 177, i. e., only during a rise or fall of potential in L25, the plunger of 177 being extended to locking position at all other times.

The sequence of operation for the exposure regulator of Fig, 12 is identical with the sequence already described for the regulator of Fig. 11 (refer Fig. 13). When a signal is received via L35 starting the exposure cycle, solenoid 176 is energized, retracting its plunger and thereby unlocking shutter 170 which thereupon rotates with drive shaft 168, uncovering the blue half of shutter 172 and effecting the blue exposure. When stop hook 174 of shutter 170 arrives adjacent stop arm 180, these elements engage, halting further rotation of shutter 170, which 18 slips on its clutch 167 out of the light path until permitted to return to locking position.

When a signal is received via L36 shifting from blue to yellow exposure, solenoid 1.78 is energized, retracting its plunger and thereby unlocking shutter 1'72 which thereupon rotates with drive shaft 168, substituting across the light path the yellow half of the shutter for the blue half and thus effecting the yellow exposure. When stop hook of shutter 172 arrives adjacent stop arm 181, these elements engage, halting further rotation of shutter 172, which, like shutter 170, slips on its clutch 167 until permitted to home to locking position.

When the termination of the total exposure is signalled via L25, solenoid 177 is energized, retracting its plunger and thereby unlocking shutter 171, which immediately trips bell crank 182, turning rockshaft 1'79 sufiiciently to disengage hooks 180-174 and 181-175, whereupon shutters 170 and 172 are freed, and all of the shutters of the regulator traverse 180 degrees back to the original positions shown in Fig. 12 where they remain until a repetition of the operating cycle.

In the employment of a focal plane regulator of the type represented in Fig. 12, the circuit shown in Figs. 9 and 9A may be utilized Without any changes, except that it is desirable to ensure that lamp 11, under the control of relay K3, does not shut off until opaque shutter 171 is completely across the light path. This can be simply accomplished by merely substituting a D. C. relay connected to power at L13 for the K3 A. C. relay shown in Fig. 9A and providing contacts K3C with a parallel-connccted capacitance of the proper value to secure the necessary delay in the same manner as now shown for relay K4. If this is done it will be understood that the present connection between 52 and L11 will likewise have to be changed to between S2 and L12 instead. These changes necessitate a correspondingly longer dwell of K, which can be achieved by substituting a larger size of capacitor Cx.

The design of exposure regulator shown in Fig. 12 has proved photographically much faster in action than the design of Fig. 11 which, in the normal case, appreciably shortens the exposure time for individual negatives and thereby increases the production rate. It may be necessary to, in effect, slow down the light response of the photofinishing paper by interposing a neutral density filter in the light path in order to reduce the intensity of illumination to a level accommodating the slower speed of the regulator of Fig. 11, but this is rarely necessary with the design of Fig. 12.

The invention of this application is adapted to use in the contact printing of variable contrast paper as well as in projection type printing, such as hereinabove described.

One commercial type of continuous strip printer operating on the contact principle is shown in Fig. 14, wherein certain modifications are indicated to permit inclusion of the apparatus of this invention. The strip printer, as modified, is semi-automatic in operation, the operator achieving two control level meter readings by manual adjustment of light level at a negative-viewing station and the printer automatically translating the operators adjustments into an exposure of the proper duration and light quality to obtain a high grade photographic print.

Referring to Fig. 14, the contact printer as supplied by the manufacturer comprises two separate stations: (A) a previewer station and (B) an exposure station. This arrangement is necessary so that the operator can obtain exposure control information from the film negative alone, which is done at the previewer station, and then in order to superimpose the negative on the photofinishing paper so that the exposure can be effected, which is done at the exposure station. Because of the fact that the previewer and exposure stations are an appreciable distance apart, the film examined in the previewer station is not immediately fed to the exposure station, but there is instead a finite length intermediate the two stations which is neither being viewed nor being exposed. In the case of 9" x 9" negative frames, the film in process in the machine comprises a first frame in the previewer station, second and third frames between the two stations and the fourth frame in the eX- posure station, the numbering convention recited being that which is adhered to in the description that follows. As seen in Fig. 14, the developed film negative is introduced into the machine as a continuous strip from a supply roll and indexed from left to right past the previewer and exposure stations in sequence to the film take-up roll, which is manually crank-operated. The photofinishing paper, which is customarily supplied in rolls of 1000 ft. length, enters the machine near the bottom and by-passes the previewer station, taking the course indicated by broken line representation around a dancer roll to the exposure station, where it is overlaid by the film negative, and thence to a powered takeup roll.

As presently designed, the contact printer incorporates total exposure control only, without any provision whatsoever for contrast control. Exposure control is achieved in the commercial machine by providing two identical, independent light sources, one for the previewer station and one for the exposure station, each of which comprises 24, 15 Watt, 115 v. incandescent frosted lamps, the operator regulating the intensity of illumination for each individual negative frame while it is in the previewer station by suitable manipulation of one of the control knobs B1, B2, B3 and B4 on the front panel of the machine and this regulation being applied later to the light source in the exposure station When the particular frame has arrived there and is properly registered for printing. Control of the intensity of illumination of all lamps correspondingly in both sources is obtained by a multiplicity of auto-transformers mounted on a rotatable drum disposed within the printer, such as General Radio Co. Variac Type No. V-5, which are adjusted by knobs B1, B2, B3 and B4 corresponding in the order named to each of the four frames numbered consecutively from the previewer station to and including the exposure station and their settings translated with a suitable delay to the exposure operation. The range of voltage regulation thus secured is approximately 85-125 volts. In addition, individual lamps may be shut off by manually opening single-pole, single-throw switches, opening coacting contacts carried by the printer drum, these switches being hereinafter collectively referred to as dodging controls. As an aid to the operator, the manufacturer provides an averaging transmission type densitometer photocell circuit (not shown), the detection cells of which are located on the side of the negative opposite the previewer light source, which circuit includes a logarithmically calibrated microammeter VMi furnishing an indication of the existing illumination for each setting of a particular control knob and the dodging controls.

The contact printing machine incorporates a number of other devices which it is not necessary to describe in detail in this application, because they have no relationship to this invention, but which are mentioned because they bear on the operation of the device as modified. These devices include a printer cycling mechanism, which is a 4 R. P. M. clocktype synchronous motor driving a cam operating a switch indexing the photofinishing paper in steps equal to the length of each negative frame plus border allowance, a manually adjustable timer (range 0.2 to 5 secs. within a precision of i2.0%) for control of the duration of exposure, a powering motor, a power supply for the electronic circuit, counters and misc-ellaneous other items.

Incorporation of this invention into the conventional contact printer hereinabove described is rather readily and inexpensively accomplished by substituting a different electro-optical negative viewing a paratus, indicated generally at 209 (Fig. 14), in the previewer station and a different light source with associated controls, indicated generally at 201 (Fig. 14), in the exposure station, also adding light quality control knobs P1, P2, P and P4 (of the same type as B B2, B3 and B4) and ammeter VMz, which may be of the same type as VMi and, of course, making the necessary electrical circuit changes hereinafter described.

Referring to Fig. 15, one embodiment of electro-optical negative viewing apparatus 2% according to this invention comprises a light-tight housing 293 within which is mounted a motor-driven scanning disk 2114, which may be of the design of scanning element 27 shown in Fig. 2, a phototube detector 205, which may be of the same type as phototube 29, and the electronic circuit elements indicated generally at 206. A mask 207, which may be of the same design as mask 36, shown in Fig. 4, is suitably supported from housing separator 2G3 with its aperture intermediate scanning disk 2% and phototube 205. The light emanating from the negative film, the limits of the path of Which are shown in broken line representation in Fig. 15, is condensed and transmitted past the scanning disk 204 and mask 2G7 to phototube 205, all of which are in optical alignment, by the lens assembly indicated generally at 2&9. A view finder, comprising a Wire 210 formed into a rectangular configuration in the region of the film negative, is secured to the underside of housing 263 as an aid to the operator in selecting special interest features of the film upon which to predicate exposure regulation. Viewing apparatus 200 is rotatably supported on pin 211 which. is in turn supported on the free-swinging support structure indicated generally at 212, Fig. 14.

Since the commercial contact printer operates by light regulation during a preset constant exposure period, it is convenient to effect the exposure to the two colors of light simultaneously, which can be accomplished by the use of the striated light filters shown in Figs. 16 and 17. The orientation of the filters with respect to the light source is not critical; however, it is preferred to dispose the individual filters which in operation move with respect to one another in adjacent relationship. Thus, filter 215, which is provided with equal width alternate striations of blue and yellow color of the same light-transmitting characteristics as the blue and yellow filters of exposure regulator 25 shown in Fig. 11, is disposed adjacent to filter 216, which is provided With equal width alternate striations of light-opaque and light-transparent nature in parallel arrangement with the striations of filter 215. Similarly, filter 217, provided with equal width alternate striations of light-opaque and light-transparent nature disposed normal to the striations of filters 215 and 216, is positioned adjacent filter 218, which is provided with a checkerboard pattern of light-transparent areas equal in width to the spacing of the striations of 217 defined by two series of light-opaque striations, one of which is disposed parallel to the striations of 217 and the other of which is normal thereto. As indicated in Fig. 16, filters 21S and 216 are movable with respect to each other in one direction in a horizontal plane, and filters 217 and 218 are movable with respect to each other in a horizontal plane but in a direction degrees from the direction of relative movement of filters 215 and 215.

The four filters 215, 216, 217, and 218 are supported loosely, one above the other, on a shelf 220 secured inside housing 221 (Fig. 17) of the exposure station light source 201 (Fig. 14), the 24 bulbs (not shown) constituting the light source being located in the space above the filters and separated therefrom by a light diffusing plate 219. Hons ing 221 is provided with an aperture 222 permitting passage of the exposing light from the source through the light filters to the film and photofinishing paper.

Relative movement between filters 215 and 216 is obtained by driving one of the filters, in this case 216 of Fig. 17, to the right or left, depending on the light quality signalled, all as hereinafter explained, by lead screw 223 driven by gear 224 keyed to the lower end of the power shaft of motor 225. A second gear 226, keyed to the upper end of the shaft of motor 225, is provided for the drive, through gearing not shown, of the tap 272 of the multiple turn potentiometer 273 of Fig. 18, this mechanical connection being indicated schematically by the broken line denoted 276. As will be hereinafter described in detail, adjustment of filters 217 and 218 with respect to each other is only infrequently required and can be accomplished by thumb screws available to the operator, or in other well-known ways not shown.

Referring to Fig. 18 the electronic circuit required for contact printing according to this invention is similar in most respects to the embodiment hereinbefore described for projection type printing, except that the necessity for delayed utilization of the information derived from negative scanning during the previewing step requires a special servo-mechanism circuit.

The phototube detector 205 may again be an R. C. A. type 929, such as previously described for detector 29, powered from the positive side of a 250 v. D. C. source through L50 with its cathode connected to ground through resistor 230. Vacuum tube 233, which may be an R. C. A. 12AX7 type, functions as a cathode follower with its grid connected to the cathode side of 205 at 231, to which L51 running to the light quality control circuit is also connected. The cathode of 233 is grounded through resistor 234 shunted by capacitor 235, which arrangement constitutes 233 as an infinite impedance peak detector. Vacuum tube 237, which may be an R. C. A. 12AT7 type, is connected as a cathode follower with its grid in circuit with the cathode of 233 and its cathode connected to the cathode of vacuum tube 241 through microammeter VM1 hereinabove described. Equal value load resistors 238 and 239 are employed for the grounding of the cathodes of 237 and 241, respectively, and the grid of tube 241 is connected as the tap 242 of variable resistor 243, which is in series with fixed resistor 244 across L50 to ground.

L51 is provided with a resistor 248, which preferably is of the order of 10 megohms resistance, as previously described for resistance 101 of the projection type printing circuit, and connects to the control grid of pentode tube 249. Pentode tube 249 and triode 252 together constitute a logarithmic amplifier, 249 having its plate connected to power lead'Lss through resistor 253, its suppressor grid connected to its cathode and its cathode grounded. The screen grid of 249 is connected intermediate series resistors 254 and 255 running from L53 to ground, and the plate of 249 is in circuit with the grid of 252 through Lst. Cathode follower tube 252, which may be an R. C. A. 12AT7 type, is powered from L: through L55, and has its cathode connected through resistor 251 to ground, and through Lss and capacitor 256 to the cathode of clamp tube 257 and the grid of detector tube 258,

each of which may be of the R. C. A. 12AT7 type.

Vacuum tubes 257 and 258 together constitute a peakto-peak detector, the grid of 257 being connected to its plate and ground, and the cathode of 258 being grounded through lead resistor 259. The cathode of 258 is in circuit with the grid of cathode follower tube 261 through Ls: which is grounded through capacitor 260, the latter serving with resistor 259 to constitute 258 as an infinite impedance detector. Triode tubes 261 and 262, each of which may be of the R. C. A. 12AT7 type, have their anodes connected directly to L53 and their cathodes connected to ground through equal value load resistors 263 and 624, respectively, microammeter VMz, which may be of the same type as VM1 being shunted across the two cathodes. The grid of 262 is permanently connected to the inside solid conducting ring of switch Sio through L53. Switch S is provided with two switch arms, Smx and 22 SIOY, affixed to the central insulated hub at right angles to each other, which sequentially make and break circuit with the four contacts $103., Silld, S100, and $101) in the order recited. The switch arm S10}: is in permanently closed circuit with the inner ring of S10 (and thus with the grid of tube 262) through sliding contact SlOXC, and switch arm SIOY is in permanently closed circuit with the outer ring of S10 through sliding contact S10v0. The outer ring of S10 is permanently connected to the grid of the first filter positioner tube 268 through L59. From the foregoing it will be understood that switch arms Srox and SIOY turn in unison in a clockwise direction as seen in Fig. 18, being driven together as a pair by a mechanical connection, indicated schematically by the broken line 265, from the timer mechanism of the printer.

Each of the four contacts Sim, 5101;, S and SIM are connected to the individual taps of multiple turn helical potentiometers regulable by light quality control knobs P1, P2, P3, and P4, respectively, these potentiometers being powered through a common resistor 266 in circuit with L50 and having their low potential ends grounded.

The servo follow-up system for this embodiment of the invention comprises the filter positioner vacuum tubes 268 and 269, which may be of the R. C. A. 6N7 type, having their plates connected to opposite ends of the coil 225C of the phase-sensitive filter positioner motor 225, and their cathodes grounded through a common load resistor 271. The grid of 269 is connected as the tap 272 of variable resistor 273, which is in circuit on one side through variable resistor 274 to the lead Leo powering coil 225C and on the other side to ground through variable resistor 275. As previously mentioned, tap 272 is suitably positioned during operation by the rota tion of filter-shifting motor 225 and gear 226, which latter operates in conjunction with a conventional mechanical connection indicated only schematically at 276, Figs. 17 and 18.

The incorporation of this invention in the commercial type contact printer hereinabovc described was predicated on making a minimum number of alterations in the printer per se and, therefore, the semi-automatic nature of the printer operation remains unchanged; however, it will be obvious to persons skilled in the art that fully automatic operation could be obtained in essentially the same manner as has already been described for the projection type printer if the design of the printer were modified to permit full automaticity.

In operation, the light source at the previewing station is constantly illuminated, while the light source at the exposure station is turned on and off by a switch integral with the printer and actuated by the printer timer for each negative frame indexed past it, so that the photofinishing paper is exposed only during the preset time interval.

Before the machine is placed in operation it is calibrated, as hereinafter detailed, in a manner similar to the calibration for the projection type machine which has already been described. The film is provided with a leader strip which is secured to the negative film take-up drum and, by manipulation of the hand crank, the operator draws the first frame of the negative into registration with the previewing station. Passage of the leader strip, and the subsequent film to which it is attached, cocks the printer timer which thereafter illuminates the light source of'the exposure station once for a preset period of time after each indexing of the film and also closes the circuit of the motor indexing fresh photofinishing paper into position for the next succeeding exposure after the exposure light source is turned off. It will be understood that, during the initial thread-up of the machine, the photofinishing paper is also provided with a leader strip, so that paper is not wasted by exposure prior to the interpositioning of a negative in the exposure station. The printer is provided with a repeat exposure switch under the control of the operator, permitting the fully auto- 2.3 matic printing of any desired number of positive prints based on the exposure settled on for the first print of the series.

With the first frame of the negative in the previewing station, the operator visually examines it and sets the dodging controls so that areas abnormally low in density will not be over-stressed in the later exposure. At the same time the operator visually selects the area of special interest of the negative, and swings the electrooptical scanning apparatus 200 (Fig. 14) into a position where the special interest area is encompassed by the viewfinder 210. The scanning apparatus operates continuously, being in circuit with the main printer power supply, and almost immediately derives a pulsating signal similar in form to that shown in Fig. 8, the instantaneous value of which is a linear function of the intensity of the light impinging on phototube detector 205. This signal is amplified in tube 233 of the total exposure control circuit and is also delivered to the logarithmic amplifier network of the light quality control circuit comprising pentode 249 and triode 252.

Any unbalance in current flow between tubes 237 and 241 of the total exposure control circuit is indicated by a corresponding unbalance in the position which the indicating needle of meter VM1 seeks with respect to its central null point, and the operator is thus enabled to adjust the total illumination by manipulation of B1 to either increase or decrease the light level to the proper degree, which is reached when the needle of VMr is restored to null position.

The signal impressed on the cathode of tube 257 and the grid of 258 of the light quality control circuit from the cathode of 252, which is coupled thereto by capacitor 256, is logarithmically proportional to the instantaneous input signal, i. e., to the instantaneous light transmission of the negative. Thus, tubes 257 and 258 together function as a peak-to-peak detector in the same manner as the double triode tube 119 shown in Fig. 9, except for reversed polarity, and the D. C. signal appearing at the point of connection of L57 with the cathode of 258 is proportional to the difference between the logarithms of the maximum and minimum light transmissions of the negative, or, as has previously been explained, to the contrast as defined in the photographic art. This signal is impressed on the grid of 261 and any unbalance in current flow between tubes 261 and 262 is indicated by a corresponding unbalance in the position which the indicating needle of meter VMz seeks with respect to the central null point. The operator is thus enabled to effect a compensation by adjustment of control knob P1, which is connected to the tap of the helical potentiometer in circuit with Sma. This completes the previewing evaluation of the first frame of the negative, and the operator indexes succeeding frames past the previewer station, making adjustments of the dodging controls and the control knobs B1, B2, B3, B4, and P1, P2, P3, and P4 in the sequence of the control knob subscript numbers. As an aid in the ascertainment of which controls are in readiness for adjustment at any given time, the printer is provided with individual signal lamps E1, E2, Es, and E4 for each tier, with switches actuated by rotation of the printer drum, the circuit to each of the lamps being closed in the sequence of their subscript numbers.

The indexing of each negative frame effects cocking of the printer timer (Fig. 18) which, for each exposure cycle, indexes the two arms 810x and Stay of switch S in unison through an angle of 90 degrees clockwise. Thus, for the first negative frame of every series of four, Smx will close the circuit from the grid of tube 262 through the inner ring of Sin, sliding contact 810m: and switch arm Srox to the tap of the potentiometer adjusted by P1. At the same time, the circuit from the tap of the potentiometer adjusted by P4 will be closed through contact Siod, switch arm SmY, sliding contact Steve and the outer ring of S10 to. L59. This relative se- 2 quence of operation is adhered to at all times, so that the fourth negative frame of any series, counted from the previewing station in the direction of the exposure station, is always printed on the basis of information previously stored in B4 and P4, while the first negative frame is being evaluated and the information on which its later exposure is predicated is impressed into the variable transformer corresponding to the first frame, by appropriate setting of B1, and into the corresponding potentiometer by appropriate setting of P1. As previously mentioned, the second and third negative frames are located between the previewing and exposure stations during the previewing of the first and the exposure of the fourth frames, and are therefore merely in transit from one station to the other, having been previously evaluated and their exposure settings made by the operator but not yet having been exposed.

When each successive negative frame arrives at the exposure station, the printer automatically establishes the circuit between the corresponding auto-transformer and the exposure light source, whereupon the total illumination level set by the operator at the previewing station is faithfully repeated at the exposure station. At the same time the light quality for the particular negative frame is established by impressing the signal from the corresponding potentiometer regulated by one of the knobs P1, P2, P3, or P on filter positioner tubes 268 and 269. If a change in light quality requirements from the previous negative frame exists, as reflected by a difference in the settings of the corresponding otentiometers, the current flow through tubes 268 and 269 is unequalized, causing unbalanced flow through the coil 225C of phasesensitive motor 224, which thereupon shifts striated filter 216 in the appropriate direction with respect to striated filter 215, increasing or decreasing the ratio of yellow to blue light accordingly and establishing the proper light quality for the negative. Motor 225 rebalances the flow of current through tubes 268 and 269 and halts its own rotation by repositioning the tap 272 of variable resistor 273 mechanically through gear 225 and the cooperating connection indicated generally at 272. When the exposure is completed the printer timer shuts off the exposure station light source and indexes the photofinishing paper a length corresponding to one print, so that the machine is in readiness for the printing of the next negative frame in the series, if the operator indexes a new frame into registration with the exposure station, or for repeat printing of the same negative, if the operator has set the repeat exposure switch of the machine for this type of operation.

Referring to Figs. 16, 17, and 18, the calibration of the contact printer embodiment of this invention is in many respects identical to that already described for the projection printing embodiment, except for certain changes arising out of the use of striated filters. The purpose of the two filters 215 and 216 is, of course, to obtain the proper ratio of yellow to blue light to meet the specific contrast requirements of a particular negative. It has been found, however, that the positioning of filter 215 with respect to filter 216 is not linearly related to contrast, and it is the objective of filters 217 and 218 to insure this linearity, so that the light compensation achieved by the adjustment of 215 with respect to 216 will be of the same high precision for all degrees of contrast encountered.

As previously described, filter 218 is formed in a checkerboard pattern of light-transmitting areas, while filter 217 is provided with a series of light-opaque lines parallel to the horizontal sides of the checkers of 218 but perpendicular to the striations of filters 215 and 216. The result is that, as 217 and 218 are moved with respect to one another, and normal to the direction of movement of 215 with respect to 216 but in the same horizontal plane as seen in Fig. 16, more light of one of the colors of filter 215 will be permitted to pass through 

